Fenton et al U.S. Pat. No. 5,476,760 discloses tabular grain emulsions with a non-uniform iodide distribution within the tabular grains that enhances photographic sensitivity. The tabular grains are formed by starting with a conventional tabular grain emulsion in which the tabular grains have a surface iodide concentration of less than 2 mole percent and, preferably, an iodide concentration of less than 2 mole percent throughout. Iodide ions are then introduced into the emulsion without concurrent silver ion introduction to displace chloride and/or bromide ions preferentially from the peripheral edges and particularly the corners of the tabular grains. This is followed by silver ion introduction, which results in the further deposition of silver halide and, surprisingly, a rearrangement of iodide ion that results in a maximum iodide concentration along the peripheral edges of the tabular grains and a lower iodide concentration within the corners of the tabular grains than elsewhere along their edges. Fenton et al demonstrates unexpectedly high levels of photographic sensitivity for these emulsions.
When a maximum peripheral iodide concentration is created by a conventional double-jet precipitation technique, the iodide concentration at the corners and elsewhere along the edges of the tabular grains is similar and the sensitivity of the resulting emulsion is significantly less than that of a comparable emulsion having the structure disclosed by Fenton et al.